DESCRIPTION (appended verbatim from investigator's abstract): Holoprosencephaly (HPE) a defect in developmental patterning of the face and forebrain is the most common malformation in early human pregnancy with an incidence of 1/250 during the first trimester. Basal cell carcinoma (BCC) is the most common form of cancer in Caucasians with 750,000 cases treated annually in the United States alone. Both conditions are associated with inappropriate activity of the Hedgehog (Hh) signaling pathway, a deficit during the embryonic period in the case of HPE and excess postnatal activity in the case of BCC. This proposal centers on the mechanisms by which the Hedgehog extracellular protein signal is sensed and transmitted to intracellular components and on the mechanisms by which a localized source of Hedgehog signal elicits a graded pattern of pathway activity in cells of a developing tissue. The Hedgehog receptor comprises two polytopic transmembrane proteins Patched (Ptc) and Smoothened (Smo) with twelve and seven transmembrane spans respectively. Genetic evidence suggests that Ptc suppresses the activity of Smo and that Hh binding to Ptc relieves this suppression allowing for activation of downstream targets. Although Smo is related to the Frizzled family of Wnt receptors and more distantly to members of the Gprotein coupled receptor family, other components of the Hh pathway have no counterparts in other signaling pathways. Molecular mechanisms of Hh receptor action thus remain unknown and experimental observations are not adequately explained by current models. We have recently developed several novel tools including various purified protein and antibody reagents a series of specific Hh receptor inhibitors sensitive cultured cell signaling assays and methods for selecting cells with high or low levels of Hh pathway activity. Using these tools together with a broadlybased experimental approach that incorporates pharmacological biochemical molecular and cell biological methods we propose: (1) to determine how components of the Hh receptor transduce the signal across the membrane; 2) to determine how Hh receptor activation affects intracellular components; and 3) to understand how a localized Hh signal elicits a graded pattern of pathway activity from cells within developing tissues such as the limb bud or neural tube.